1. Field of the Invention
This invention relates to a push-up pin of a semiconductor element pushing-up device in a die bonding apparatus, a semiconductor element pushing-up device in a die bonding apparatus, and a method for separating a semiconductor element in a semiconductor element pushing-up device of a die bonding apparatus.
2. Description of the Related Art
The process for bonding a semiconductor element obtained by subjecting a semiconductor substrate to the dicing process on a lead frame in the method of manufacturing a semiconductor device is called a die bonding process.
The conventional die bonding apparatus is shown in FIG. 1.
The die bonding apparatus is mainly constructed by a portion for taking up one semiconductor element, a portion for moving the taken-up semiconductor element onto a lead frame, and a portion for carrying the lead frame.
The portion for taking up the semiconductor element includes a wafer ring 2 for fixing a semiconductor substrate 1 obtained after semiconductor elements are subjected to the dicing process with the semiconductor substrate attached to the adhesive sheet, an XY table 3 for carrying the wafer ring 2, a camera 4 set above the XY table 3, and a semiconductor element pushing-up device 30 disposed below the XY table and used for pushing up the semiconductor element from the rear surface side of the adhesive sheet by use of a pin or pins so as to separate the semiconductor element from the adhesive sheet.
The portion for moving the semiconductor element onto the lead frame 12 includes an element suction head 10 for taking up the semiconductor element separated from the adhesive sheet and moving the semiconductor element to a position correcting stage 11, and the position correcting stage 11 for correcting the position of the semiconductor element, and a bonding head portion 8 for holding the semiconductor element by use of a collet and carrying the semiconductor element from the position correcting stage 11 onto the lead frame.
The portion for carrying the lead frame includes a lead frame supplying portion 5 for supplying a lead frame, a lead frame carrying portion 6, a paste supplying portion 7 for supplying adhesive onto the lead frame, and a lead frame receiving portion 9.
The portion for taking up the semiconductor element is explained in more detail with reference to FIGS. 2A, 2B, and 3A to 3D.
FIG. 2A is an enlarged top plan view showing the semiconductor substrate 1 on the semiconductor element pushing-up device 30, and FIG. 2B shows a cross section taken along the line IIB--IIB of FIG. 2A and the construction of peripheral devices of the semiconductor element pushing-up device 30.
FIGS. 3A to 3D are cross sectional views for illustrating the operation of the semiconductor element pushing-up device 30.
The semiconductor element pushing-up device 30 includes a backup holder 15, push-up pins 17, pin holder 19, pin holder driving device 31, control device 32, and vacuum device 20.
The backup holder 15 is a vacuum chamber having through holes 18 formed in the upper surface thereof 16 and vacuum suction force is applied to an adhesive sheet 14 on the upper surface of the backup holder 15 by use of the vacuum device 20 connected to the vacuum chamber so as to fixedly hold the adhesive sheet 14 on the upper surface thereof.
Semiconductor elements 13 are attached to the adhesive sheet 14.
The pin holder 19 capable of receiving a plurality of push-up pins 17 is inserted into the vacuum chamber of the backup holder 15 and the pin holder 19 is driven in the vertical direction by the driving device 31 shown in FIG. 2B.
The control device 32 controls the operation of the driving device 31 to drive the pin holder 19 in the vertical direction.
As shown in FIGS. 3A to 3D, if the pin holder 19 is moved upwardly in the state shown in FIG. 3A, the push-up pins 17 pass through the through holes 18 formed in the upper surface of the backup holder 15 to push up the semiconductor element 13 on the adhesive sheet 14 (FIG. 3B).
Since the adhesive sheet 14 is fixedly held on the backup holder 15 by vacuum suction force, the semiconductor element 13 is separated from the adhesive sheet 14 and taken up by suction of the element suction head 10 (FIG. 3C).
After this, the pin holder 19 is moved downwardly and the vacuum suction is released.
Then, a new semiconductor element 13 is placed on the backup holder 15 with the adhesive sheet 14 disposed therebetween in such a position that it can be taken up by suction of the semiconductor element suction head 10 (FIG. 3D).
Generally, the bonding force between the semiconductor element 13 and the adhesive sheet 14 depends on the property of the adhesive of the adhesive sheet 14 and the area of the semiconductor element 13. However, in the above-described conventional element pushing-up device 30, the conditions of the shape of the tip end portion of the push-up pin 17, the traveling distance in the vertical direction, the moving speed, and the vacuum suction pressure are kept constant irrespective of the above factors.
Recently, the adhesive strength between the semiconductor element 13 and the adhesive sheet 14 increases with an increase in the area of the semiconductor element 13 and it becomes difficult to separate the semiconductor element 13 from the adhesive sheet 14 under the constant condition as in the conventional case.
Therefore, in order to separate the semiconductor element 13 from the adhesive sheet 14, it is necessary for the push-up pins 17 to push up the semiconductor element 13 with extremely large force. In this case, there occurs a problem that the adhesive strength between the adhesive and the rear surface of the semiconductor element 13 becomes larger than the adhesive strength between the adhesive and the adhesive sheet 14 by application of the above force, and adhesive is left behind on the rear surface of the semiconductor element 13 which is separated from the adhesive sheet 14 or the adhesive sheet 14 is broken and left behind on the rear surface of the semiconductor element 13.
The above phenomenon is explained more in detail below.
FIG. 4 is a side view showing the push-up pin 17 of the conventional element pushing-up device 30.
The push-up pin 17 has a cylindrical portion 17a to be engaged into the pin holder 19, a conical coupling portion 17b, and a tip end portion 17c having a curved surface with the radius R of curvature.
The angle .theta. of circumference of a sector formed by a curved surface portion of the tip end portion 17c on a cross section taken along a line passing the central axes of the above portions is less than 180.degree..
FIG. 5 is an enlarged view showing a state in which the push-up pin 17 pushes up the semiconductor element 13. The push-up pin 17 pushes up the semiconductor element 13 while expanding the adhesive sheet 14 and adhesive 23.
In FIG. 5, since the adhesive sheet 14 and adhesive 23 are disposed between the tip end portion 17c of the push-up pin 17 and the rear surface of the semiconductor element 13 in an A zone, the adhesive sheet 14 and adhesive 23 are difficult to expand. On the other hand, since the adhesive sheet 14 and adhesive 23 are not set in contact with the rear surface of the semiconductor element 13 in a B zone, they can easily expand in the B zone than in the A zone.
Thus, the adhesive sheet 14 and adhesive 23 cannot expand equally in the A zone and in the B zone.
As a result, as shown in FIG. 5, the adhesive sheet 14 and adhesive 23 become extremely thin particularly on the boundary line S between the tip end portion 17c and the conical coupling portion 17b. Therefore, the coupling strength between the adhesive 23 on the tip end portion 17c and the adhesive 23 on the conical coupling portion 17b or the coupling strength between the adhesive 23 and the adhesive sheet 14 is lowered on the boundary line S and the adhesive strength is lowered.
As a result, the adhesive 23 in the A zone is cut apart from the adhesive in the B zone on the boundary line S and attached to the rear surface of the semiconductor element 13 or the adhesive sheet 14 is broken and attached to the rear surface of the semiconductor element 13 via the adhesive 23.
If the semiconductor element having the adhesive or adhesive sheet 14 thus left behind on the rear surface thereof is received into a plastic package, a crack may be caused in the package by the thermal stress when it is mounted on a circuit board, thus making the device defective.
Particularly, in a package with a structure in which the rear surface of the semiconductor element is directly covered with sealing plastic as in the LOC (Lead On Chip) structure which is frequently used recently, cracks may easily occur.
Further, there occurs a problem that since the load applied to the push-up pin 17 is increased, the push-up pin 17 will break through the adhesive sheet 14.
If the push-up pin 17 breaks through the adhesive sheet 14, the tip end portion of the push-up pin 17 is brought into direct contact with the rear surface of the semiconductor element 13, but since large force is applied to the tip end portion of the push-up pin 17 as described before, damage such as crack, scratch or mark of the push-up pin 17 is formed on the rear surface of the semiconductor element 13.
If the semiconductor element 13 having the damage formed on the rear surface thereof is set into a plastic package, a crack may be caused in the semiconductor element 13 by the thermal stress when it is mounted on a circuit board, thus making the device defective.
Semiconductor elements 13 having the damage formed on the rear surface thereof and semiconductor elements 13 having no damage were set into respective plastic packages, then subjected to the soldering process by infrared heating and mounted on circuit boards, and the defective rates thereof were compared. The result of comparison was that no defective device was contained in 139 semiconductor elements having no damage on the rear surface thereof but ten devices were found defective in 550 semiconductor elements having the damage formed on the rear surface thereof.
Thus, the probability that the semiconductor elements having the damage formed on the rear surface thereof become defective is high.
Further, in the conventional element pushing-up device 30, the process is continuously effected even after the adhesive sheet 14 is broken. At this time, as indicated by broken lines in FIG. 2A, the backup holder 15 applies vacuum suction force to the adhesive sheet 14 in a wide range covering not only the semiconductor element 13 to be subjected to the pushing-up process but also an area in which the semiconductor element 13 was already separated and taken up in the preceding pushing-up process.
Therefore, if the adhesive sheet 14 is broken in the preceding pushing-up process, vacuum suction force is also applied to the broken portion and the pressure will leak via the broken portion of the adhesive sheet 14 and cannot be sufficiently lowered so that the adhesive sheet 14 cannot be fixedly held. As a result, the semiconductor element 13 cannot be separated from the adhesive sheet.
Further, if the push-up pin 17 breaks through the adhesive sheet 14, the tip end portion of the push-up pin 17 is brought into direct contact with the rear surface of the semiconductor element 13 with large force, and in this case, the tip end portion of the push-up pin 17 may be damaged. At this time, the conventional element pushing-up device 30 continues the operation, the adhesive sheet is successively broken and the rear surface of another semiconductor element 13 is damaged by the damaged tip end portion of the push-up pin 17.
In the conventional element pushing-up device 30, since breakage of the adhesive sheet 14 cannot be detected until the die bonding process for a preset number (or one lot) of semiconductor elements is completed, a large number of defective devices will be produced when the breakage of the adhesive sheet is detected.
Thus, in the conventional element pushing-up device, the conditions of the shape of the tip end portion of the push-up pin 17, the traveling distance in the vertical direction, the moving speed, and the vacuum suction pressure are kept constant irrespective of the property of the adhesive of the adhesive sheet 14 and the area of the semiconductor element 13 and the operating state of the push-up pin 17 is not controlled according to different conditions.